Some of the earlier cars (with suicide doors ) had the radiator under the front bumper. It is the same one basically that was used under the car. Sometimes, this radiator was used exposed and sometimes it had a small shroud. I have both the shroud and the small radiator for my car. I am not sure if the shroud was a later addition, or if it was simply left off for racing purposes. I can't imagine the radiator under the car lasting very long, as it hangs rather low.
Some time in the mid sixties (I think) Abarth flared the rear fenders slightly with a small bulge. You can see this in the photo in this thread. I have a set of fiberglass replica ones, but am not sure if the originals were fiberglass or if Abarth modifed the steel body. Also, on the later cars with wide rear flares, the wheel wells were modified and enlarged for the really wide tires that were used.
If it were me, I would never use the large (late) radiator shroud and large fender flares on an early (suicide door) car. It is simply not historically correct. I have seen quite a number of books on Abarths that have photos of cars that are supposed to be Abarths, but have the late shroud and flares on an early car. I have also seen a few cars like this on ebay. These are either fakes, or Abarths that have been butchered...
I do think it is okay to roll the fender lips a bit on the early cars to provide clearance for modern 13" tire sizes. I don't think you can find skinny tire widths anymore to fit in these cars.
Does anyone know how the early, small radiator shrouds were attached? They wrap over the stock bumpers, and I'm not sure if they fasten to the bumper or body.
Gil